A walking assist device which assists walking motion of a human by applying torques to the knee joint of the human is being developed. For example, the patent document 1 (Japanese Patent Application Publication No. 2006-314670) discloses the walking assist device suitable for the user who is not able to move one of the legs properly. In this description, a leg which the user can entirely control is referred to as a “sound leg” and a leg which the user can not properly control at least one joint is referred to as an “affected leg” hereafter. Further, a part of the leg between the knee and the ankle is referred to as a “lower leg” and a part of the leg between the hip joint and the knee is referred to as an “upper leg”. The walking assist device in the patent document 1 measures a motion pattern of the sound leg by a sensor and applies torque on the joint of the affected leg so that the motion pattern of the affected leg follows the motion pattern of the sound leg.